Conformations of Semi-flexible Polymers

    The typical conformations of semi-flexible polymers depend on the spatial scale, on which they are observed. On molecular scales, these polymers behave as semi-rigid rods that keep their spatial orientation up to the so-called persistence length. On larger length scales, the polymers loose their orientation and attain random coil conformations. This behavior implies a renormalized, scale-depending bending stiffness that decays to zero on large scales [1].

    When a weak stretching force is applied to a semi-flexible polymer, it primarily acts to reduce the amplitude of thermally excited shape fluctuations. A strong stretching force, on the other hand, probes the molecular bonds between the monomers or subunits of the polymer. The full range of forces is covered by the semi-flexible harmonic chain model [2], which was originally introduced for the hydrocarbon tails of lipid molecules [3]. When a compressive force is applied, the semi-flexible polymer undergoes a buckling (or Euler) instability at a critical buckling force, which is also affected by thermal fluctuations [4, 5].

    A rather interesting class of semi-flexible polymers is provided by oligo- and polysaccharides. Even though the backbone of an oligosaccharide assumes an essentially straight conformation in the absence of external forces, it can be easily bent by forces of physiological magnitude as revealed by atomistic Molecular Dynamics simulations [6, 7].


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